The first part of this proposal intends to explore the molecular mechanism of "homing" hemopietic progenitor cells (HPC) to the marrow stroma and specifically to test the hypothesis that this phenomenon is mediated by lectin-like membrane receptors recognizing sugar residues of membrane glycoproteins. Using differential agglutination technique, the surface of HPC will be explored for the presence of lectin receptors and receptors recognizing biologically relevant sugars. The pattern of appearance and disappearance of these receptors in relation to the lineages of differentiation will be studied. To demonstrate that this type of inteaction is responsible for the binding of HPC to stromal cells long-term marrow cultures (LTMC) will be grown in the presence of inhibiting concerntrations of synthetic neoglycoproteins and total cell and HPC production will be studied as a function of time in both the adherent layer and the supernate. Synthetic neoglycoproteins will also be used to inhibit in vivo homing of transplanted marrow cells after lethal doses of total body radiation. In the second part of the proposal, transendothelial transport of hematologically relevant circulating proteins in the marrow will be studied with transferrin (TF) as a prototype protein. To do so marrow endothelial cells will be isolated and their surface will be explored for the presence of TF receptors at 4 degrees using 125I- TF. At 37 degrees 125I-TF will be chased to demonstrate that internalization is followed by externalization. The released substance will then be studied by a variety of biochemical methods to find out if the endothelium alters the molecule in the course of its transport. Transendothelial transport will also be studied in situ using perfusion with collodial gold labeled protein and radiolabeled protein followed EM autoradiography. Through these studies we hope to learn if marrow endothelium has a regulatory function with regard to the uptake of hemopoietically relevant circulating proteins.